Since their advent in the 1960s, the use of biosensors has become widespread. A biosensor is a device that couples a biological recognition element (e.g., an enzyme or antibody), with a transducer (e.g., an electrode or photodiode), to convert biochemical information into an electric signal.
FIG. 1 shows the action of a glucose biosensor that includes an enzyme coated electrode 1 to which a voltage potential is applied. The biosensor of FIG. 1 is an example of amperometric detection in which a voltage is applied to the electrode 1 which causes a particular analyte (the substance being measured) in the sample to oxidize or (i.e., give up electrons to the electrode). The oxidation cause a current 3 to be generated which can then be detected and analyzed. The potential at which the analyte oxidizes is called the “oxidation potential” of the analyte.
Generally speaking, the term “redox potential” is used to indicate the potential at which an analyte is either oxidized or reduced. In the biosensor of FIG. 1, glucose (“GLU”) reacts with the enzyme and transfers electrons to the enzyme, converting it from its oxidized state to its reduced state. Some other electron shuttle (in an oxidized form) reacts with the enzyme to turn it back over to its oxidized state. The electron shuttle then becomes reduced in the process (taking electrons from the reduced enzyme). The reduced electron shuttle is what is oxidized at the electrode. One example of such an electron shuttle is oxygen, being reduced to hydrogen peroxide. There is also a family of electron shuttles called mediators that are used in many commercial glucose test strips that perform this function instead of relying on oxygen.
Using the technique of amperometry, selectivity towards one of several analytes in a sample is achieved by applying the redox potential of that analyte. Thus, in FIG. 1, a sufficiently high potential is being applied to oxidize the reduced electron shuttle, and the resultant current 3 detected by the electrode depends on the concentration of the reduced electron shuttle, which in turn depends on the glucose concentration in the sample. (It should be noted that in actuality a mediator agent associated with the glucose is reoxidized and reacts with the reduced enzyme. The concentration of the reduced mediator is directly indicative of the concentration of glucose in the sample. For the sake of simplicity glucose will be referred to as the analyte being oxidized with the understanding that it is in fact the reduced mediator that is the actual analyte detected at the electrode.)
Electrochemical biosensors are an attractive offering due to their low cost and ease of manufacture, however other blood chemicals, such as ascorbic acid (vitamin C), acetaminophen (“TYL” in FIG. 1), and uric acid can interfere with the biosensor action resulting in erroneous readings. FIG. 1 shows the effect of the interferent ascorbic acid (“C”), in which a molecule of ascorbic acid 5 has diffused through the enzyme layer, been directly oxidized by the electrode 1, and generated a current 7.
Thus, when a sample contains several analytes, all with overlapping redox potentials (that is, where the redox potentials of several analytes are within the same ranges and thus all give rise to a redox current at the same applied electrode potential), the selectivity of the electrode diminishes. The current generated at the electrode results from all analytes from the sample that can be electro-oxidized or electro-reduced at the given electrode potential, resulting in a sensed current that includes unknown components of each analyte, thereby resulting in diminished electrode selectivity and incorrect concentration readings. Testing for an analyte without accounting for analytes with overlapping redox potentials will result in inaccurate readings.
FIGS. 2-4 graphically illustrate the foregoing problem, with respect to hydrogen peroxide (the target analyte) and ascorbic acid (the interferent). As shown in FIGS. 2 and 3, increasing amounts of each of ascorbic acid and hydrogen peroxide (x-axis) when applied with the same DC amperometric voltage of 600 mV generate an increase in the sensed current (y-axis). Calibration curves 9 and 11 are determined from current readings 13 and 15, respectively.
Because the redox potentials overlap at 600 mV, false readings result as shown in FIG. 4, when testing for hydrogen peroxide. The tester should read a concentration of 1 mM, as indicated by dashed line 17. The tester instead falsely generates readings 19 showing increasing amounts of hydrogen peroxide when in fact increasing amounts of ascorbic acid are added to the measured sample containing a constant amount of hydrogen peroxide.
Other interferences that commonly plague biosensors include cross-reactivity with other sample components, physical deterioration or fouling of the sensor, or background noise. Efforts to overcome the foregoing shortcomings of biosensors have traditionally been to use physical or chemical enhancements to the device such as using chemical mediators or perm-selective membranes. However, mediators can contribute to increased background noise and membranes add unnecessary production costs while reducing the sensor's overall sensitivity.